How to Select a VNA Architecture for Efficient and Accurate Characterization of mmWave Devices

July 21, 2020 Anritsu Company

July 21, 2020

There are several key applications pioneering the use of millimeter wave (mmWave) communications technology, ranging from 5G New Radio (5G NR) and automotive radar to tactical communications and unmanned aerial vehicle (UAV) imaging and communications. Device characterization testing on devices operating at these higher frequencies poses challenges for design engineers in need of generating accurate models for circuit simulation.  

To aid these engineers in gaining design confidence, advanced vector network analyzer (VNA) technology (figure 1) has been developed that provides a superior method than traditional concatenated waveguide bands. These new architectures utilize broadband vector network analyzer (VNA) and mmWave connector technology to enable broadband testing to 220 GHz with a single test unit and setup for improved device modeling.

ShockLine NLTL
Figure 1

Component Testing Importance

testing of amplifiers, mixers, isolators/circulators, filters, attenuators, transmission lines (interconnect), transformers, and other mmWave components is necessary to refine the design and manufacturing process, pass certification testing, and ensure production quality. It must be conducted in a manner to ensure the highest accuracy over the frequency range of the device under test (DUT).

Design engineers rely on characterization testing to determine device performance parameters and create device models for electronic computer-aided design (ECAD) systems during the development process. How well the characterization testing is performed can often influence how many refining iterations are needed during development. It impacts the length of the design cycle and time-to-market.

Traditional Test Methods

Until recently, there have been two main methods of performing testing for wafer and semiconductor die probing – concatenated waveguide band approach and broadband measurement techniques. The concatenated waveguide band approach requires test instruments with frequency-extending hardware that uses waveguide interconnects that typically exhibit much less loss than coaxial options.

As waveguides are intrinsically banded interconnects, several frequency extender units – and possibly several test instruments – are needed to conduct tests to hundreds of gigahertz. One method is to use a VNA that operates to 110 GHz with a 1-mm coaxial interconnect, then integrate a frequency extender with D-band waveguide, followed by another frequency extender using WR5 (140 GHz to 220 GHz) waveguide.

Another factor with this approach is that back-end-of-line (BEOL) parasitic circuit elements and probe interconnect resistances and capacitances can adversely affect measurement accuracy. Figure 2 shows how it can impact on-wafer measurements certainty and device model accuracy.

RF Wafer Probe
Figure 2

Calibration must also be performed for each setup. Due to the wear on the landing pads, calibration structures, and probes, repeatability of each test is diminished and uncertainty increases. Different calibration standards may be used, increasing the challenge of successfully de-embedding the test setup. Conversely, with a true broadband measurement system (DC to the maximum frequency), very accurate and consistent de-embedding can be performed to the device level and with fewer touchdowns.

NLTL-based VNAs

Anritsu has developed a VNA architecture that overcomes the inherent limitations of VNA systems based on down conversion technologies, such as step-recovery diode (SRD), in high-frequency designs. The Anritsu non-linear transmission line (NLTL) sampling VNA technology is coupled with monolithic reflectometer designs and components to enhance broadband testing.

NLTL-based broadband VNAs (figure 3), such as the VectorStar™ family, provide signal quality, reduce cost, enable extreme bandwidth, mitigate isolation calibration issues, and enable more compact VNAs that can be placed closer to DUTs. NLTL-based VNA architectures rely on continuous wave signals that then undergo fall-time compression via non-uniform NLTLs to create narrow gating pulses for sampling receivers. Distributed harmonic generation is enabled by using the “harmonic growth” characteristics of NLTLs.

VectorStar NLTL-based broadband VNA
Figure 3

The result is extremely high RF sampler bandwidth with a single sampler covering a much broader frequency range with lower noise floor than an SRD-based sampler or many classical harmonic mixers. Utilization of a high LO range also minimizes noise introduction and image response issues.

NLTL VNA technology also results in a coupling structure with optimum raw directivity. This optimized raw directivity aids with the modeling process by improving measurement stability and lengthening the time between calibrations. Other broadband VNA technologies exhibit dramatic negative raw directivity, making them much more susceptible to environmental effects. Reduced calibration and measurement stability is the result.

By utilizing NLTL technology, the VectorStar VNAs benefit from RF and LO frequency scalability and high channel-to-channel isolation without suffering degradation from poor isolation experienced by other VNAs. Additionally, the monolithic design and compact nature of NLTL-based VNAs reduce the temperature variations between the reflectometers and constituent components.

Engineers reap benefits from this VNA architecture when characterizing their devices used in mmWave designs. The VNAs have enhanced calibration integrity, measurement accuracy, and measurement repeatability. Because of the reduction in size of the mmWave NLTL module, on-wafer probes may attach directly to the module, thereby eliminating the need for interconnect cables. The broadband VectorStar VNA thereby minimizes interconnection losses due to long and lossy transmission lines associated with typical mmWave testing environments.

You can learn much more about VNA technologies best suited for mmWave device characterization by downloading a new application note entitled New VNA Technologies Enable Millimeter-Wave Broadband  Testing to 220 GHz.

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